Showing papers in "Plant and Soil in 1981"

Experimental determination of nitrogen kinetic isotope fractionation: Some principles; illustration for the denitrification and nitrification processes

Abstract: A few principles relative to the presentation and use of nitrogen stable isotopic data are briefly reviewed. Some classical relationships between the isotope composition of a substrate undergoing a single-step unidirectional reaction, are introduced.

Techniques and experimental approaches for the measurement of plant water status

Abstract: Living cells need to be more or less saturated with water to function normally, but they are usually incomplete in this desirable condition. The two basic parameters which describe the degree of unsaturation, i.e. the plant water deficit are (i) the water content and (ii) the energy status of the water in the cell. The water content is usually expressed as relative to that at full saturation, i.e. the relative water content or water saturation deficit, and the energy status of the water is usually expressed as the total water protential. Although the two parameters are linked in such a way that the total water potential decreases as the water content decreases, the relationship between the two, variously known as the moisture release curve, water potential isotherm or water retention characteristic, is not unique but varies with species, growth conditions and stress history (Slatyer, 1960; Jarvis and Jarvis, 1963; Altmann and Dittmer, 1966; Noy Meir and Ginzburg, 1969; Ludlow, 1976; Jones and Turner, 1978). Thus for completeness, both the water content and energy status of the water in plant tissue need to be measured. The total water potential (7') at any point in the plant can be partitioned into its components: the osmotic potential (re), turgor pressure (P), matric potential (z) and gravitational potential. As the gravitational component of the total water potential is only 0.01 MPa m- 1 (0.1 MPa = 1 bar), it can be neglected, except in very tall trees (Conner et al., 1977). For cells in equilibrium with their surroundings the total water potential is the same throughout the system, i.e. in the wall, cytoplasm, organelles and vacuole. However, the components of the total water potential may be quite different: in the vacuole the total water potential arises largely from osmotic and turgor forces, whereas in the wall, it arises largely from matric forces and to a small degree from osmotic forces. Thus the total water potential of a plant cell is given

Nitrogen and plant production

Abstract: Green plants play a unique role among living organisms through their ability to reduce carbon in photosynthesis. While reduced carbon provides the energy source for all life, nitrogen must be viewed as the central element because of its role in substances such as proteins and nucleic acids which form the living material. Proteins serve as enzyme catalysts in metabolic pathways, as structural elements of cytoplasm and membranes and as carriers in transport functions. Nucleic acids provide the means for codification, storage and translation of genetic information. In those and other organic materials, nitrogen appears in a chemically reduced state. Such organic nitrogen constitutes 1.5 to 5 per cent of the dry weight of higher plants, and 80 to 90 per cent of that is in protein. Despite the great abundance of dinitrogen gas in the atmosphere, the element is commonly deficient in agricultural soils in the oxidized (nitrate) and reduced (ammonium) forms which can be absorbed by plants. In aerated soils, bacteria quickly transform ammonium ions to nitrate, and the oxidized form is the principal in one plant nutrition. As a result of nitrogen's critical roles and low supply, the management of nitrogen resources is an extremely important aspect of crop production. It has also been an area for intensive research at all levels from yield behavior to cell physiology. As agronomists, we are concerned particularly with designing strategies to increase the efficiency of nitrogen use in crop systems. This chapter focuses on aspects which are related to the uptake and assimilation of nitrogen into amino acids and proteins, and their subsequent interaction in growth and development. These events are highly dynamic and complex, and are not yet fully understood. Our intent here is to provide a broad review of those subjects for crop physiologists, geneticists and agronomists. We begin with the biochemical and cellular level and integrate towards field performance.

Changes of pH across the rhizosphere induced by roots

Abstract: Plants that absorb nitrogen as NO3 − tend to raise the pH in the rhizosphere. Those absorbing nitrogen as NH4 + or N2 lower the pH. The change in pH near the root surface may be calculated approximately from the H+ or HCO3 − efflux and radius of the root; and the pH buffering capacity, moisture content, initial pH and pCO 2 of the soil. An accurate equation, solved numerically, also takes account of root hairs, mass flow and slow acid-base reaction in the soil. The pH at the root surface will often differ from the pH a few mm away by 1–2 units.

Alley cropping maize (Zea mays L.) and leucaena (Leucaena leucocephala Lam) in southern Nigeria

Abstract: A maize-leucaena alley cropping system was studied on a N-deficient sandy Apomu soil series (Psammentic Usthorthent) at Ibadan in the forest zone of southern Nigeria from 1976 to 1980. In this system maize was grown in 4 m width alleys between Leucaena hedge rows.

Water dynamics in the soil-plant-atmosphere system

Abstract: The water problem in agriculture is related both to weather and to the reserves of water in the soil that are available to plants. Water dynamics in the soil-plantatmosphere system concerns the capacity of the soil water reservoir, its depletion and replenishment, and its efficient management for crop production. The concept of the soil as a reservoir for water is appealing and useful. Since only a small amount of water can be stored in crop plants relative to the rate of transpiration through them, it is the storage of water within the soil pores that permits transpiration to continue for several days without recharge by rainfall or irrigation. However, water storage in the soil is not similar to that in a bucket. Some water may drain out of the root zone, and not all water remaining in a drying soft can be taken up by the plant as rapidly as it is needed because it is held too tightly by soil particles. Although methods of determining the capacity of the soil water reservoir available to the plant are not exact, the concept permits calculations of the soil water balance and its impact on crop production. Water-balance calculations using computers are becoming more common. There should be more emphasis on water-balance technology in the future because it is needed for accurate estimation of crop yields, early warning about food shortages, better farm management, reliable irrigation scheduling and water-resource planning, etc. Because of these urgent needs, it is important to develop models of the water balance that are as general as possible so that local calibrations are eliminated or at least minimized. Models should also not depend on the input of weather records that are difficult to obtain. The dynamics of the soil-water balance requires separate understandings of the atmospheric, plant and soil-water factors which affect the soil water balance. These factors are interdependent but will be discussed separately for simplicity.

Plant-water relations and adaptation to stress

Abstract: Many of the effects of water deficits on the growth and yield of plants are clearly evident throughout arid and semi-arid regions of the world. The dramatic increases in yields of cereals during the past 40 years in regions where a regular supply of water is assured through rainfall or irrigation, and the smaller increases where water supply is limited, have only heightened awareness of the constraint that a limited water supply places on improvement of yield. In the United Kingdom where rainfall provides a plentiful supply of water for crop growth, yields of wheat have climbed steadily over the past 35 to 40 years at an average annual rate of 67 kg ha 1 year 1 (Fig. la), while yield increases in Sweden and Mexico under non-limited water conditions appear to be even more spectacular than those in the United Kingdom (Evans, 1978). By contrast, in Australia and Syria where wheat is principally a water-limited dryland crop, yield increases have been much smaller: in Australia wheat yields have increased by ! 3 kg ha 1 year1, while in Syria there has been no significant increase over the same period (Fig. la). In defense of Australian breeders and agronomists, it must be pointed out that there has been a rapid expansion of wheat production since 1956 into newer and more marginal areas (Fig. lb): this contrasts with the static amount of area harvested over the same period in Syria and the United Kingdom, Nevertheless it is clear that in water-limited environments, less advantage can be taken of improved varieties and higher fertilizer use than in more temperate environments or regions in which irrigation can be provided. A second feature of cereal production in the dryland Mediterranean regions, exemplified by Australia and Syria, is the large year-to-year variation in yield per unit area (Fig. la): an analysis by Russell (1973) showed that the year-to-year variation in yield of wheat in Australia was much greater than that in USA. This is undoubtedly a function of the large year-to-year variation in rainfall in Mediterranean-type environments (Harris, 1979). In spite of smaller yield increases in dryland Mediterranean areas compared to those obtainable in more temperate regions, the Australian example in Fig. 1 does indicate that yield increases in dryland cereal production are possible in

Effects of a soil fulvic acid on the growth and nutrient content of cucumber (Cucumis sativus) plants

Abstract: Cucumber (Cucumis sativus) plants were grown in Hoagland solution to which 20 to 2000 ppm of a soil fulvic acid (FA) were added. The addition of 100 to 300 ppm of FA produced highly significant increases in the growth and development of above and below ground plant parts, in the uptake of nutrient elements (N, P, K, Ca, Mg, Cu, Fe and Zn), and in the formation of numbers of flowers per plant. Effects of adding 500 and more ppm of FA were less beneficial.

Proteoid root morphology and function inLupinus albus

Abstract: Current theories of phosphorus uptake by plants imply that they can augment diffusion to their root axes by the development of abundant root hairs or mycorrhizas. Some phosphorus efficient plants have root morphology with multi-branched roots and localised regions of densely packed root hairs, which we suggest is better suited to the retention of substances exuded by the roots than uptake of substances moving to the root by diffusion. Evidence of substantial exudation by the proteoid roots ofLupinus albus is presented.

Optimizing the use of water and nitrogen through breeding of crops

Abstract: Although many crops are grown in ICARDA's region, wheat and barley dominate. This paper concentrates on these two crops. Much breeding effort and physiological research has been expended on wheat and barley and many of the principles which have emerged from this work can be applied to less important crops. Breeding of both wheat and barley over the past 20 years has lifted dryland yields modestly in certain Mediterranean-type environments (e.g. southern Australia, Israel). The methodology of this progfam has been traditional, with heavy reliance upon empirical yield testing. The physiological basis of this genetic progress is not understood, although at least two of the factors involved are better phenological adaptations (i.e. a more appropriate flowering date) and increased yield potential (i.e. greater yield in the absence of water stress). Many other traits have been identified as important for increasing dryland yield but, with the possible exception of the presence of awns and a conservative or low tillering growth habit, claims are not supported either by good experimental evidence or unanimity of opinion. Hence we must concentrate in our discussions on the experimental approaches needed to provide evidence to justify trait screening or to improve breeding strategies in any other way. I will review these approaches, as they have been used for wheat and barley, and attempt some suggestions. Elucidation of the physiology of performance under drought takes two general forms; this separation probably reflecting the philosophy of the investigator as much as anything else. Pragmatists attempt to proceed from yield differences to underlying process differences. I call this the black box approach. At the same time, theorists wish to predict differences in yield from an understanding of processes. This procedure is exemplified in the ideotype strategy.

Dilute hydrochloric acid as an extractant for available zinc, copper and boron in rice soils

Abstract: Because zinc deficiency is a widespread disorder of wetland rice and copper deficiency may occur with it, 0.05M HCl was compared with 0.1M HCl, EDTA and DTPA as an extractant for available zinc and copper. It was also compared with the reflux method for boron assay. Thirty-three wetland rice soils were analyzed for zinc and copper by the four methods. Rice was grown on the flooded soils, scored visually for zinc deficiency, and the plants analyzed for zinc and copper content. In the boron study, 53 soils were extracted by the reflux method and the 0.05M HCl procedure. Rice was grown on the flooded soils, scored visually for boron toxicity, and the plants analyzed for boron content. Fourteen of the 16 soils on which rice showed zinc deficiency gave <1.0 mg/kg Zn by the 0.05M HCl method but values far in excess of the critical limits by the other methods. The r values for available and plant zinc were: 0.05M HCl (0.88**); 0.1M HCl (0.55**); EDTA (0.43**); and DTPA (0.31ns). Twelve of the zinc-deficient soils gave<0.1 mg/kg Cu by the 0.05M HCl method but values exceeding the critical limits by the EDTA and DTPA methods. The r values for available and plant copper were: 0.05M HCl (0.74**), 0.1M HCl (0.64**), EDTA (0.28ns), and DTPA (0.20ns). The critical limit of 1.0 mg/kg by the 0.05M HCl extraction was confirmed for zinc deficiency and a tentative value of 0.1 mg/kg for copper deficiency proposed. The 0.05M HCl method separated boron-toxic soils from non-toxic soils and gave a better correlation (r=0.91**) between available and plant boron than the reflux extraction (r=0.84**). The toxic limit by the 0.05M HCl method was provisionally set at 4 mg/kg.

Soil nutrient status of hill agro-ecosystems and recovery pattern after slash and burn agriculture (Jhum) in north-eastern India

Abstract: The present study deals with fertility changes in agro-ecosystems where vegetation is removed by slash and burn procedures, the land is planted to crops (cultivated) for one year, and then left to revegetate naturally for upto 50 years (forested fallow, here after referred to as ‘fallow’) before the entire cycle (locally called ‘Jhum’) is repeated. A comparison has been made between three jhum cycles of 30, 10 and 5 years. Depletion in soil carbon continued throughout the cropping period of one year and extended upto a 5 year fallow. This could be one of the reasons against a short jhum cycle, alongwith a similar pattern in depletion of nitrogen. Available phosphorus build up in the fallows also starded only beyond a 5 year fallow period with rapid increase in 10, 15 and 50 year fallows. Cationic concentration in the soil also rapidly declined in the early phases of regrowth of vegetation. This decline was most pronounced for potassium due to the fact thatDendrocalamus hamiltonii is a heavy accumulator of this nutrient. Since this bamboo species dominates the fallow upto about 20 years, potassium build up in the soil was observable only at this stage. It is suggested that this species plays an important role in conservation of this nutrient. In a 50 year fallow, low levels of calcium and magnesium were maintained with rapid depletion of both with depth which is in contrast to that of potassium and phosphorus. In general, short jhum cycles of 5 year permit only low levels of soil fertility with very poor recovery during the fallow period. The significance of these results are discussed.

Climate, soil and land resources in North Africa and West Asia

Abstract: The international institutes concerned with agricultural research appreciate the need for inventories of physical resources relevant to the use of land for crop production. Such inventories can be used to plan research programmes and in the interpretation of records from the field. This paper reviews the climatic and soil resources in the region served by ICARDA within which the term 'Mediterranean' has been applied to a wide range of environments.* In North Africa the countries covered are Morocco, Algeria, Tunisia, Libya, Sudan and Egypt; in West Asia, they are Jordan, Saudi Arabia, People's Democratic Republic of Yemen, Yemen Arab Republic, Oman, United Arab Emirates, Qatar, Bahrain, Lebanon, Israel, Syria, Turkey, Iraq, Kuwait, Iran and Afghanistan. In a recent report from the Food and Agriculture Organisation (FAO, 1978), the methodology for assessing potential land use is based on the following procedures: i selection and definition of the type of land utilization, i.e. crop and product, production type, input level; ii compilation of a climatic inventory including phenological requirements and the response of photosynthesis to temperature and radiation; iii assembly of information on the soil requirements of the crop; iv compilation of a quantitative climatic inventory (1:5 million scale) based on climate (characterizing temperature differences) and length of growing periods (characterizing period when water and temperature permit crop growth); v assembly of a soil inventory, by countries, from the FAO/Unesco Soil Map of the World (1:5 million scale); vi overlay of the climatic inventory on the soil map and area measurement of resultant climate: soil units, the so-called agro -eco log ica l zones;

Rock-phosphate mobilization induced by the alkaline uptake pattern of legumes utilizing symbiotically fixed nitrogen

Abstract: Soybean and alfalfa were grown on sand and soil to which P was added in the form of finely ground rock phosphates. When the legumes depended on NO3 as N source, more anionic than cationic nutrients were absorbed. This resulted in a pH increase in the growth medium and in very low availability of P added as rock phosphate. When, however, the legumes made use of symbiotically fixed N, more cationic than anionic nutrients were absorbed leading to an acidification of the growth medium and an ensuing mobilization and higher availability of the rock phosphates. Symbiotic N fixation which initiates the chain of reactions leading to an increased availability of rock phosphate-P is dependent on photosynthate supply and on the availability of phosphate. Therefore, in a separate experiment it was investigated whether a priming effect exerted by a small quantity of added easily soluble phosphate, could enhance the availability of rock phosphate-P to legumes. Results obtained indicated that easily soluble phosphate might indeed be effective in this respect.

Salinity tolerance of the tree legumes: mesquite (prosopis glandulosa var. torreyana, p. velutina and p. articulata) algarrobo (p. chilensis), kiawe (p. pallida) and tamarugo (p. tamarugo) grown in sand culture on nitrogen-free media

Abstract: Sand culture pot experiments were carried out with Proposis seedlings in the greenhouse on a nitrogen free nutrient solution with increasing levels of sodium chloride. All species tolerated a 6,000 mg/l salinity with no reduction in growth.P. velutina was the only species that poorly tolerated the 12,000 mg/l salinity level.P. articulata, P. pallida, andP. tamarugo tolerated 18,000 mg/l NaCl with little reduction in growth and grew slightly in a salinity (36,000 mg/l NaCl) greater than seawater. This is the first legume known to grown in salinities equivalent to seawater.

Natural (non-pathogenic) death of the cortex of wheat and barley seminal roots, as evidenced by nuclear staining with acridine orange

Abstract: Nuclear staining with acridine orange was used to assess cell viability in the cortex of wheat and barley seminal roots from glasshouse and field experiments. Results from this method correlated well with nuclear assessments made in unstained or Feulgen-stained roots, and other evidence is presented to support the validity of the method. The pattern of root cortex death (RCD) was similar in wheat and barley and consistent over a wide range of conditions. Behind the extending root tip and zone of nucleate root hairs, nuclei disappeared progressively from the outer five (of six) cortical cell layers of the root axes, starting in the epidermis. Stainable nuclei remained in the sixth cell layer, next to the endodermis, and in most cell layers around the bases of root laterals and in a small region immediately below the grain. The onset of cell death was apparently related more to the age of a root region than to its distance behind the root tip, and it was not closely correlated with endodermal or stelar development assessed by staining with phloroglucinol/HCl. The rate of RCD was much faster in wheat than barley in both glasshouse and field conditions, and faster in some spring wheat cultivars than in others in the glasshouse. RCD occurred in sterile vermiculite and perlite and was not enhanced by the presence of soil microorganisms; nor was it enhanced in soil by the addition of the non-pathogenic fungal parasitesPhialophora radicicola var..graminicola orMicrodochium bolleyi. RCD is suggested to be endogenously controlled by the amount of photosynthate reaching the cortex. Its implications for growth of soil microorganisms and especially for growth and biological control of root-infecting fungi are discussed.

Effect of phosphorus and zinc on the growth and phosphorus, zinc, copper, iron and manganese nutrition of rice

Abstract: A greenhouse experiment was conducted to study the effect of phosphorus and zinc application, in three lowland alluvial rice soils (Haplustalf) on the growth of rice and the concentration of phosphorus, zinc, copper, iron and manganese in shoots and roots. The results showed that application of phosphorus and zinc significantly increased the dry matter yield of shoots, grains and roots. Application of phosphorus caused a decrease in the concentration of zinc, copper, iron and manganese both in shoots and roots. Application of zinc also similarly lowered the concentration of phosphorus, copper and iron, but increased that of manganese in shoots and roots. The decrease in the concentration of the elements in the shoots was not due to dilution effect or to the reduced rate of translocation of the elements from the roots to tops. This has been attributed more to the changes in the availability of the elements in soil resulting from the application of phosphorus and zinc.

Effect of azospirillum inoculation on some growth parameters and n-content of wheat, sorghum and panicum

Abstract: The potential of the nitrogen fixing bacteriumAzospirillum brasilense to enhance development and increase growth of several gramineae was investigated. In both sterilized and non-sterilized systems heading and flowering occurred earlier in the inoculated plants as compared to the noninoculated ones. Total shoot and root weights, total-N content, plant height and leaf length were significantly increased by inoculation.

A comparison of penetrometer pressures and the pressures exerted by roots

Abstract: Previous work is reviewed in which the ratio of the pressures required for soil penetration by roots and penetrometers are compared. It appears that this ratio can vary from about 2 to 8 depending on conditions. However, there is very little experimental evidence and most of the work has been inferential. Direct measurements are reported for the stresses exerted by a 1 mm diameter penetrometer probe and by the roots of pea seedlings when penetrating Urrbrae fine sandy loam. Six soil conditions were used: (non-weathered remoulded soil cores + artificially weathered remoulded soil cores + undisturbed field clods) × (confined + unconfined cores or clods). The confinement treatment was to test for any effects of additional restraint to cylindrical root expansion. The weathering and field clod treatments were to test the hypothesis that root elongation is facilitated by tensile failure ahead of the root tip. The principal conclusions are as follows. The laboratory weathering treatment reduced the soil tensile strength by 25%. This resulted in a small but significant reduction in the pressure for root penetration into confined cores. Compared with remoulded non-weathered cores, field clods had a 2 to 3 fold greater penetrometer resistance and a 50% lower tensile strength. The force required for root penetration into unconfined field clods was only 10% greater than for unconfined non-weathered cores. For the former (which is closest to field conditions) the penetrometer had to exert a pressure 5.1 times greater than a root tip in order to penetrate the soil. Penetrometer penetration pressure was independent of probe diameter in the 1–2 mm range in the soil used. Core confinement restricts root radial expansion and modifies the penetration force of metal probes and plant roots. On the basis of the new results it is tentatively concluded that soil tensile failure can facilitate penetration by roots.

Relation between root respiration and root activity

Abstract: It is generally assumed that the major part of the energy derived from respiration in higher plants is used for growth and maintenance processes [2,4,6]. In roots a considerable part of the respiration is used for uptake and transport of ions [3], so that for a quantitative approach to the relation between root activity and root respiration three processes have to be taken into consideration: root growth, uptake and transport of ions and maintenance processes.

Influence of potassium and manganese on growth and uptake of magnesium by soybeans ( Glycine max (L.) Merr. cv. Bragg)

Abstract: Soybean (Glycine max (L) Merr. cv. Bragg) seedlings were grown in nutrient solutions to evaluate the response to manganese nutrition as affected by potassium supply. In solutions containing 275 μM manganese, increasing the solution concentration of potassium from 1 mM to 10 mM alleviated symptoms of manganese toxicity, decreased manganese concentrations in the leaves and increased dry matter yields of the plants. The reduction in manganese toxicity was brought about by a reduced rate of root absorption of manganese at high potassium supply levels. Increasing the supply of either potassium or manganese decreased the leaf concentration of magnesium although there were no apparent symptoms of magnesium deficiency in any treatment. The reduced concentration of magnesium in the leaves was due to effects of potassium and manganese on the rate of root absorption of magnesium. Under manganese deficiency conditions, growth was reduced and manganese concentrations in plant parts were very low; there was no effect of potassium supply when manganese was absent from the nutrient solution.

Biological control ofRhizoctonia solani in strawberry fields byTrichoderma harzianum

Abstract: Trichoderma harzianum preparations was used in two successive field experiments in commercial strawberry nurseries and fruiting fields. Disease severity ofRhizoctonia solani in daughter plants was reduced by 18–46 % in the treated nursery plots. Infestation of nursery soil with the pathogen, as tested by planting beans in soil samples was reduced by the Trichoderma treatment by up to 92% as compared to the untreated control. A rapid decline of the disease was observed in soil fromT. harzianum treated plots, successively planted with bean seedlings. More isolates ofTrichoderma sp. antagonistic toR. solani, were found in the infested field as compared to the non infested one.Trichoderma harzianum treated plants, transferred to the commercial field gave a 21–37% increase in early yield of strawberries. A combined treatment in the nursery and in the fruiting field resulted in a 20% yield increase as compared to control plots.

Flooding responses in Zea mays L.

Abstract: This study was undertaken to determine the specific mechanisms of flooding injury inZea mays L., especially the role of water deficit. Maize plants in soil were artificially flooded in greenhouse and field, both with and without exchanging the soil solution with new water and nutrients. Plants in solution culture were rapidly stressed by replacing aeration gas with nitrogen. In all cases of measurable short term response, root and leaf growth rates decreased within 1 to 12 hours, and stomatal resistance increased 2 to 3 days later. Both growth rates and stomatal resistance recovered spontaneously during the flooding period. Over the long term (more than a week), growth rates were inhibited only when the soil solution was not periodically changed and Kjeldahl measured nitrogen deficiency was evident in the leaves. None of the above effects was associated with high root resistance to water uptake, plant water deficit, or a change in the osmotic balance of the leaves. The only water stress associated with the experiments was found immediately after draining plants that had been flooded for 2 weeks. In this instance, the preflooding root systems appeared damaged.

The effect of lime on nitrogen mineralization as measured by grass growth

Abstract: The effect of rates of lime and nitrogen on the growth of ryegrass (Lolium perenne) was measured in a pot experiment using two yellow-brown earth steepland soils (pH 5.1 and 5.3). Nitrogen or raising the soil pH above 5.8–6.0 markedly increased growth on both soils. Negative lime×N interactions on both soils together with other data indicated that an increase in the rate of mineralization of N was the major effect of lime. Numbers of bacteria and fungi as measured by plate counts were not increased by liming and hence the increased rate of mineralization of N was attributed to increased microbial activity.

Influence of waterlogging and lime or organic matter additions on the distribution of trace metals in an acid soil

Abstract: A study has been made of the influence of waterlogging on the distribution of trace metals between the various reservoirs in which they are held in a soil. Selective extractants have been used to remove metals held in 4 ways: soil solution and exchangeable; specifically adsorbed by inorganic sites; adsorbed or chelated by organic sites; adsorbed onto oxide surfaces. Waterlogging over a period of 16 weeks resulted in the release of both manganese and iron from the organic — and oxide — bound reservoirs to the soluble, exchangeable and inorganic reservoirs. Addition of both 1% dried grass (as an actively decomposing organic material) and 1% CaCO3 to the soil resulted in an acceleration of the metal redistribution. For manganese, selective extraction methods accounted for the distribution of all the metal in the reservoirs studied. In the case of iron however, there appeared to be some release from a reservoir not being extracted.

Accession, transformation, and loss of nitrogen in soils of the arid region

Abstract: Nitrogen is a key element in improving crop productivity throughout the world. It has attracted considerable research attention both as a plant nutrient and as an environmental pollutant and several reviews have appeared (Bartholomew and Clark, 1965; Nielsen and MacDonald, 1978; West and Skujins, 1978; NRC, 1978). Nearly all of the research on nitrogen as a plant nutrient has been conducted in irrigated or humid ecosystems where spectacular improvements in crop yield have been achieved through the use of large amounts of N. In arid and semi-arid regions, limited water resources and low crop productivity have discouraged the widespread use of N and consequently limited the research interest in this element. However, with an ever-increasing demand for food, it is now realized that arid and semi-arid regions will need to be exploited to the fullest extent. This may be achieved only through a better understanding of the various components of these ecosystems and their interactions (Russell, 1977). With the introduction of improved crop varieties which are responsive to fertilizer, applied nitrogen has become an important input in these ecosystems. Nitrogen is one of the most complex and mobile plant nutrients. Fig. 1 is a general diagram representing the various cyclic N-processes operating in a soil-plant system. In the internal nitrogen cycle, immobilization and mineralization are continuously causing changes in the mineral nitrogen reserves of the soil. Simultaneously, mineral N may be converted to gaseous forms due to denitrification or release of ammonia from the soil solution. An external cycle between the soil and the atmosphere depletes the soil's gaseous nitrogen content by emission of ammonia, nitrous oxide, or nitrogen gas, while enrichment of the nitrogen in the soil takes place through biological nitrogen fixation, nitrogen deposition, or nitrogen fertilization. Mineral nitrogen in the soil can also be depleted through the uptake of nitrogen by the crop, whereas the return to the soil of the non-harvested crop will add to the organic nilrogen pool. Finally, alternate wetting and drying conditions in the soil will tend to move mineral nitrogen up and down the profile. Where excessive wetting prevails, mineral nitrogen may leach beyond the reach of the crop roots. The

Factors affecting growth and nodulation ofHippophaë rhamnoides L. ssp.Rhamnoides in soils from two successional stages of dune formation

Abstract: To explain the decline of Hippophae scrub in the vegetation succession in the dunes of The Netherlands, the growth and nodulation of Hippophae plants grown in pots, using soil from an early stage (site AH) and a post-optimum stage (site HP), were investigated. In HP-soil nodulation, yield, and the nitrogen and phosphorus content of test plants were always lower and the number of necrotic nodules and the dry matter content were always higher than in AH-soil, even after inoculation with crushed nodules and the addition of a nutrient solution. Plants in HP-soil also had darker roots, less root hairs, a higher number of short lateral roots and a higher percentage of dead roots than those in AH-soil. These characteristics of adverse growth conditions disappeared upon ignition or gamma-irradiation of HP-soil. Possible explanations of these results are discussed. The degeneration of Hippophae scrub cannot be ascribed to the age of the plants, the absence of sufficient infective endophyte particles or to abiotic factors such as unfavourable physical (particle size) or chemical soil conditions but is caused by biotic factors. No indications were obtained that plant-pathogenic fungi and bacteria are involved. HP-soil in contrast to AH-soil, however, contained large numbers of the nematodeLongidorus sp., a species known to cause root deformations. The conclusion was that this nematode is one of the biotic factors involved in the degeneration of the Hippophae scrub. This degeneration is due to a restriction of the root system resulting in a low phosphate uptake, a low nodulation capacity and, as a consequence, a low nitrogen content. The results demonstrate that biotic soil factors are important in influencing succession in higher plant communities.

Heavy-metal accumulation in crops grown on sewage sludge amended with metal salts

Abstract: Poplar (Populus euramericana ‘Robusta’), oats (Avena sativa L. ‘Leander’), maize (Zea mays L. ‘Ona 36’), English ryegrass (Lolium perenne L.), butter head lettuce (Lactuca sativa L. ‘Reskia’), spinach (Spinacia oleracea L. ‘Subito’) and French beans (Phaseolus vulgaris ‘Prelude’), were grown in pots with pure sewage sludge (pH 6.7), amended with Cd, Cr, Cu, Ni, Pb and Zn acetates, either added singly or in combination, to study metal effects on plant growth and metal uptake. Phytotoxic metal doses varied with metal and plant species, increasing in the order Cd

Effect of lime and phosphorus applications on concentrations of available nutrients and on P, Al and Mn uptake by two pasture legumes in an acid soil

Abstract: Effects of increasing rates of lime and phosphorus addition on concentrations of available nutrients in soil and on P, Al and Mn uptake by two pasture legumes, lotus (Lotus pedunculatus Cav.) and white clover (Trifolium repens L.), were studied in a pot experiment using a highly leached acid (pH 4.2) soil.